Hydraulic circuits are widely used on mobile machines such as mowers, construction equipment, agricultural tractors and the like for powering one or more "work functions." Hydraulic systems are ideal for this operating environment at least since, unlike mechanical drive systems, they are not restricted to straight mechanical drive lines. That is, hydraulic motors, cylinders can be mounted in out-of-the-way places and fed by fluid pumped through flexible hose-like hydraulic lines.
Another advantage of a hydraulic circuit is that output power may be readily controlled. There is a wide choice of control valve configurations and new ones are continually being developed.
It is not unusual for a hydraulic circuit to be required to power several different functions, sometimes simultaneously. And different functions have differing characteristics that must be recognized as the circuit is being configured. For example, where the machine is equipped with power steering, the circuit must be arranged so that as to two or more circuits powered from a particular pump, the power steering circuit has priority.
Other functions such as the hydraulic cylinder used to raise and lower the 3-point hitch on a tractor or the hydraulic cylinders used to fold and unfold the extension wings of a planter are used only occasionally in what might be described as "setup" operations. Still other functions, while not critical to operator safety or vehicular control like power steering, are nevertheless required to operate more-or-less continuously. The hydraulic motor powering a crop sprayer pump is an example.
As examples of known hydraulic circuits, U.S. Pat. No. 5,313,795 (Dunn) depicts a hydraulic circuit having two fixed-displacement pumps. The first powers the steering system on a priority basis and also powers the brake and/or implement hydraulic system. The second pump powers a brake on a priority basis and also powers the implement hydraulic system.
U.S. Pat. No. 5,615,553 (Lourigan) discloses a two-pump circuit in which the first pump powers, on a priority basis, a torque-converter transmission, brakes and the like. Such pump also powers auxiliary valves if the priority needs have been met. The second pump powers auxiliary valves and pump output flows are joined under certain operating circumstances.
As evidenced by the Dunn and Lourigan patents, it is not unusual to include two hydraulic pumps in a circuit. Of course, the number of pumps is a function of their flow outputs and circuit flow demands. Sometimes the flows of two pumps are combined. And since a hydraulic circuit is a dynamic system involving trapped liquid under pressure and involving columns of liquid which move and stop upon demand (much like water in a household water system), pressures (including pressure "spikes") and flows can interact with one another and produce some undesirable results.
The new hydraulic circuit described below is specially configured to isolate intermittently- and continuously-operated functions from one another. The circuit also isolates the rather-sensitive control mechanism of a variable displacement pump from the vagaries of pressure spikes and the like which may occur elsewhere in the circuit.